The present invention relates to a configuration of the electrolyte membrane for a fuel cell; a configuration of the electrolyte membrane electrode assembly for the fuel cell in which the sealing configuration, arranged on the peripheral aspect of the electrolyte membrane for the fuel cell, has been particularly improved; and a fuel cell employing this improved configuration of the electrolyte membrane-conjugated electrodes.
The basic principle of the polymer electrolyte membrane type fuel cell is to remove the electricity from the reactive energy created by water synthesis through a chemical reaction via an ion exchange membrane, which is a polymer membrane, based on exposure of oxidation gas, such as air, on one side and fuel gas on the other side, respectively.
The generating element of the fuel cell is formed on both sides of the polymer electrolyte membrane, which selectively transports hydrogen, wherein the polymer electrolyte membrane is sandwiched between a pair of electrodes, each comprising a catalyst layer (anode catalyst layer and cathode catalyst layer, respectively) having a carbon powder carrying a platinum group metal catalyst as the main ingredient and a gas diffusion layer, which has both gas permeability and electron conductivity. This basic element for generating electrical energy is called the MEA (membrane-electrode-assembly). The circumference of each electrode is lined with gaskets, which surround the polymer electrolyte membrane on both sides to prevent leakage of fuel gas, which is supplied to one electrode, and oxidation gas supplied to the other, so as not to allow mixing of the two kinds of gases. These gaskets and the MEA are generally unified by a method of thermal compression bonding.
The basic configuration of this fuel cell is provided with gaskets and an MEA, which are interleaved between the anode separator, which has a fuel gas flow channel, and a cathode separator, which has an air flow channel.
Generally, each separator is provided with a pair (optionally multiple pairs) of penetrating holes (manifolds), which are designed for connecting the grooves on the separator plates directly to the ends of lines, which are used for supplying fuel gas, oxidation gas, and cooling water to each separator. This configuration allows, for example, fuel gas to be sent from the fuel gas supply manifold and divided among the fuel gas flow channels of the anode separators, where it is consumed in the process of the fuel gas flowing from the flow channels into the MEA cell reaction process. The excess fuel gas is disposed into a fuel gas exhaust manifold. The oxidation gas follows the same process. The fuel cell should be cooled with cooling water, etc. to maintain a preferred temperature, because of the heat that accompanies the generation of electrical energy. For this reason, a cooling member incorporating a cooling water flow channel is provided on the backside of one in every three cells. This MEA, the separator plates, and the cooling member are piled up one after another, and the assembly of unit fuel cells is fixed at both ends of this fuel cell stack, which is interleaved into end plates via a collector plate and an insulator and affixed with fastening bolts. The layered fuel cell is simply called a “stack”.
FIG. 1 depicts a perspective view of a conventional fuel cell stack (partially exploded), and FIG. 2 is a cross sectional view along lines II-II in FIG. 1. Only part of the stack configuration is shown in FIGS. 1 and 2, for sake of clarity.
FIG. 3 shows in plan view one example of an air flow channel pattern of a cathode separator, which was provided in the conventional stack, and FIG. 4 shows in plan view one example of a hydrogen flow channel pattern on an anode separator, which was provided in the conventional stack. Additionally, FIG. 5 is a plan view, which shows an MEA and a gasket configuration that accommodates the cathode separator shown in FIG. 3 and the anode separator shown in FIG. 4. FIG. 6 is a cross sectional view along line V-V of FIG. 5.
As shown in FIGS. 1-10, hydrogen manifold 3, water manifold 4, and air manifold 5, are formed at both ends of the rectangular anode separators 1 and the cathode separators 2. The anode separator 1 and the cathode separator 2 are connected by pressure welding via O-ring 8 (water cooling surface sealing material). The water-cooling flow channel 9 is formed on one side of the anode separator 1, which connects with the cathode separator 2, and the cathode separator 2 connects with the anode separator 1, respectively. Additionally, airflow channel 10 located on the distal side of anode separator 1 from cathode separator 2, and the hydrogen flow channel 10 on the distal side of cathode separator 2 in relation to anode separator 1 are formed, respectively.
Exemplary patterns of this airflow channel 10 and the hydrogen flow pathway 11 are shown in FIGS. 3 and 4, respectively.
Referring to FIGS. 5 and 6, MEA 13 is arranged in a configuration having an ion exchange membrane 12, anode catalyst layer 13, and cathode catalyst layer 14, which are formed on either side of the ion exchange membrane 12, and gas diffusion layers 7. In the stack anode catalyst layer 13 and cathode catalyst layer 14 are interleaved between anode separator 1 and cathode separator 2. Gaskets 6 are provided to sandwich the outer edges of ion exchange membrane 12, around gas diffusion layers 7 for the hydrogen and air, which are supplied to gas diffusion layers 7, so as not to become mixed or leaked. As mentioned above, the MEA 15 and gaskets 6 may be unified by thermo-compression bonding.
As already mentioned, an important object of a gas seal in the fuel cell is to prevent gas cross leakage, which is more specifically a mixing phenomenon of fuel gas and oxidation gas that are mixed because of an imperfect seal. This phenomenon has a close relationship with the seal configuration of the fuel cell. When the flow channel has a meander shape with a direct connection to the manifold (see the example shown in Japanese Patent No. 2711018), the gasket located in the outer edge part of the MEA is supported on only one side of the separator with this flow channel part, as the configuration shows clearly. Consequently, a leakage route occurs in two places due to the gasket drooping down to the flow channel side of the electro-conductive separator plate located in or around the gas manifold. One path of leakage is the gasket, which is supported on one side, and which droops down to the flow channel of the separator. Consequently, space is created in the separator on the other side, which forms a leakage path. The other cause is the leakage path which is created by the separation of the membrane and the gasket, when the gasket is deformed as mentioned above, because the connecting strength of the gasket and the membrane is very weak. The usual membrane is made with denatured fluorinated resin, although the usual gasket is formed by attachment to the membrane.
This cross leakage is quite harmful to the function of the fuel cell. Considering the leakage of fuel gas into the oxidation gas manifold through the above-mentioned leakage path next to the manifold of one cell, all of the oxidation gas, which is supplied to all of the cells, includes fuel gas, because the gas manifold is shared by all of the cells according to the stacked configuration. Consequently, leakage not only causes a drastic decrease in voltage, but the polymer membrane becomes damaged because of catalytic combustion of fuel gas, which essentially should have been consumed by the fuel cell reaction with the oxidation gas on the air electrode side. As a result, destruction of the cell finally ensues, because both electrode gases become mixed through perforation of the cell membrane, which suffers severe damage.
Considering these circumstances, various kinds of cross leakage prevention methods have been devised. However, these devices were only designed with the shape of the separator side (see the example shown in Japanese unexamined patent publication, No 2002-203578).
FIG. 7 is a plan view showing a part of the conventional cathode separator configuration, which was devised as mentioned above, and FIG. 8 is a cross sectional view along the line VII-VII in FIG. 7. The method of construction, in which the gasket is supported by insertion into the hydrogen flow channel of a different piece for bridge part 11A, which is next to the hydrogen manifold 3 of the cathode separator 2 shown in FIGS. 7 and 8, is the simplest and most commonly used method.
The so-called submarine system (see the example shown in Japanese unexamined patent publication No. 2002-83614) is a related idea, as shown in FIGS. 9 and 10, which provides full support for the gaskets in the proximity of the manifold by providing a gas channel from each backside of the cell utilizing a water cooling phase of the separator.
There are many proposals designed to prevent cross leakage using a single material for the gasket-sealing configuration (see the examples shown in Japanese published patent applications Nos. HEI7-501417, HEI8-45517, and HEI8-507403).
The method of construction which is illustrated in FIGS. 7 and 8 requires two arrangements on both the fuel gas side and the oxidation gas side. Consequently, from the viewpoint of accuracy and the difficulties associated with carrying out the complex assembly itself, it is hard to consider this construction for mass production of a polymer membrane type fuel cell, which requires the buildup of hundreds of cells in a stack.
Although the above-mentioned submarine method has an excellent design to eradicate cross leakage, it does create the problem of volume increase (causing a decrease in the electric density). Thus, it absolutely must have one cooling surface per cell, because of the built-up configuration of the stack. It also poses difficulties in cost reduction, because a low cost molded separator cannot be used. Thus, it is technically difficult to use a conventional molded separator with this kind of shape that is equipped with small perforations. Consequently, when the most common shaped separator is used, the gasket itself must have rigidity, resistance to bending, and sufficient elasticity to act as a sealant. However, the highly elastic material, which shows an excellent sealing effect, cannot resolve the above-mentioned problem of droopiness, because it performs with insufficient rigidity, and easily “creeps” because it lacks mechanical strength. Conversely, a material which demonstrates excellent accuracy in terms of dimension and mechanical characteristics, cannot be expected to have a sealing effect. In sum, the above-mentioned gasket-sealing configuration using a single material poses difficulties in usage.